A person's sleep can be assessed with a polysomnogram (PSG), which is a multi-channel procedure carried out in a sleep laboratory. Typically, the procedure requires labor-intensive technician support, resulting in an expensive process. The studies are typically performed for a single night in a Sleep Laboratory and sometimes also during the day to study daytime sleepiness, e.g., with a Multiple Sleep Latency Test (MSLT). The results of the sleep study are primarily (i) indices related to apnea events, such as an Apnea-Hypopnea Index (AHI), and (ii) sleep staging outputs that indicate the stages of sleep that occurred.
Sleep stages reported as a result of a sleep study often follow either the Rechtschaffen and Kales (R&K) scoring system or the American Academy of Sleep Medicine (AASM) system established in 2007. In the R&K system the stages of sleep are S1, S2, S3, S4, REM (Rapid Eye Movement), and Wake. In the AASM format, S3 and S4 were combined into a single stage, N3, with the stages of sleep being N1, N2, N3, REM, and Wake. However, a typical PSG requires multiple EEG (electroencephalogram) channels, an EOG (electrooculogram), an EKG (electrocardiogram), an EMG (electromyography), and analysis of data from other sensors. As a result, a PSG can be a rather invasive procedure and is typically administered for only a single session or two.